1. Field of the Invention
The present invention relates to an apparatus and method for converting between different video formats, and more particularly, to an apparatus and method for converting between different video formats to enable visual communication between remote users of different television systems.
2. Description of the Related Art
There are three major standard television systems available today: NTSC, PAL, and SECAM. NTSC is used in the United States and Japan, while PAL and SECAM mainly in European nations. For comparison, FIG. 18 shows some key specifications of those three different video standards, including: frame frequency, the number of effective picture elements (pels or pixels) per line, and the number of effective lines per frame. Although PAL and SECAM seem to share some common parameters, SECAM is actually incompatible in operation with PAL.
As noted above, different video formats are used in different groups of nations, and to solve this incompatibility problem in international visual communications, it is necessary to introduce some standardized video coding formats. Typical video formats for such purposes are H.261 and H.263, recommendations from the International Telecommunications Union-Telecommunications Standards Sector (ITU-T). They provide two common video formats called xe2x80x9cCommon Intermediate Format (CIF)xe2x80x9d and xe2x80x9cQuarter Common Intermediate Format (QCIF).xe2x80x9d Some of their key specifications are shown in FIG. 19 to make a comparison in terms of the number of picture elements per line, the number of lines per frame, and frame frequency. In this table of FIG. 19, the symbol xe2x80x9cYxe2x80x9d represents luminance signals, and xe2x80x9cCrxe2x80x9d and xe2x80x9cCbxe2x80x9d denote color difference signals.
FIG. 20 shows a simplified system structure for international digital video communications between two countries using different television system standards. This video communications system interconnects an NTSC system 2 and a PAL system 4 by using CIF as a vehicle for relaying digital video signals via a communications satellite 3. To convert the video format, a video signal processor 200a is disposed at the NTSC side, and another video signal processor 200b at the PAL side. The former processor 200a comprises a format converter 201a and a codec (coder/decoder) 202a, while the latter processor 200b comprises a format converter 201b and a codec 202b. Such video signal processors 200a and 200b are implemented as part of videoconferencing stations or videophone terminals, for example.
Consider that the NTSC system 2 attempts to send video information to the PAL system 4. The format converter 201a in the video signal processor 200a converts incoming NTSC video signal into a CIF video stream, which conforms to the common format for communications. This CIF video stream is then encoded by the codec 202a and transmitted to the communications satellite 3 via a radio transmitter (not shown). The radio signal retransmitted by the communications satellite 3 reaches a receiver (not shown) attached to the video signal processor 200b. The video signal, now in the form of an electrical signal, is supplied to the codec 202b for video decoding, where the original CIF video stream is reconstructed. The format converter 201b converts this CIF video stream to a PAL video signal for use in the PAL system 4. It would be understood that video signal transmission in the opposite direction (i.e., from PAL to NTSC) can be achieved in a similar fashion.
As described above, conventional visual communications systems use the CIF as a common format to transport video information to remote sites. That is, the sender encodes video signals after converting from its local format to the CIF format, and the receiver converts from the CIF format to its local format after decoding reception signals. In such international video communications, conventional systems resolve the difference in frame frequencies by subsampling or duplicating video frames. More specifically, when a video signal having a certain frame frequency has to be converted to a lower frequency (e.g., 30 Hz to 25 Hz), the source frames are subsampled, or decimated, at predetermined intervals. On the other hand, when a video signal has to be converted to a higher frequency, some frames are sent twice to adjust the frame frequency.
However, the above-described video conversion techniques introduce some unnatural effects into objects"" motion in a video, because of the discontinuity of frame pictures. Viewers may perceive the drop or duplication of frames as awkward motion of objects, particularly when the video contains rapid motion such as a soccer ball flying in a parabola.
Taking the above into consideration, an object of the present invention is to provide an apparatus for converting between different video formats to enable visual communications between users of incompatible television systems, which maintains the smoothness of object motions by preventing unnatural effects from being introduced in the process of video format conversion.
Further, another object of the present invention is to provide a video format conversion method for visual communications between users of incompatible television systems, which maintains the smoothness of object motions by preventing unnatural effects from being introduced in the process of video format conversion.
To accomplish the first object, according to the present invention, there is provided an apparatus for converting from a first video signal in a first video format to a second video signal in a second video format that is incompatible with the first video format. This apparatus comprises: a frame interpolator which produces interpolated frames from the first video signal by using motion vectors obtained therefrom; and a video signal generator which produces the second video signal from the interpolated frames.
To accomplish the second object, according to the present invention, there is provided a method of converting from a first video signal in a first video format to a second video signal in a second video format that is incompatible with the first video format. This method comprises the steps of: producing interpolated frames from the first video signal by using motion vectors obtained therefrom; and producing the second video signal from the interpolated frames.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.